1. Field of the Invention
The present invention relates to a system for playing back information recording disc, and more specifically to a system for playing back a recording disc which carries digitally processed information. The invention further relates to a system, to be used in the disc player system, for demodulating a PCM digital signal recorded on an information recording disc.
2. Description of Background Information
As a recording disc on which digital signals are recorded, the so called digital audio disc, that is, the compact disc (designated as the CD generally), is known in which audio signals are treated by a predetermined digital modulation process and recorded on the disc in the form of a pulse train. Recently, in addition, a type of video disc has been developed in which an audio signal which is digitalized by a predetermined digital modulation method and converted to a pulse train signal is superimposed on FM (frequency modulated) signals representing a video signal and the audio signal. The recording is performed with the thus produced recording signal.
The details of this type of video disc (referred to as the LDD hereinafter) are, for instance, described in Japanese Patent Application No. 58-45780.
In the recording process of this type of disc, two audio signal channels are processed and two carrier signals of 2.3 MHz and 2.8 MHz, as audio carrier signals, are modulated in frequency by the audio signals, respectively. In addition, the video signal is converted in frequency so that the sync tip frequency is at 7.6 MHz, Pedestal level is at 8.1 MHz, and White Peak is at 9.3 MHz respectively.
The above audio signals are also digitalized and transformed to a pulse train signal according to a modulation method such as the PCM (Pulse Code Modulation) process.
The pulse train signal takes the form of a signal suited for recording, obtained by a process such as the EFM (Eight to Fourteen Modulation) process for example, and the frequency spectra of the audio signal are transformed to frequency components of the pulse train which comprises pulses having a width in the range 3T through 11T.
In this expression, T represents the bit period of the PCM signal, 3T corresponds to 720 KHz, and the maximum width 11T corresponds to 200 KHz.
The pulse train signal having this characteristic is superimposed on the video main carrier signal at a level less than 1/10 of that of the main carrier and then sliced and amplified at around the zero-crossing point. The pulse width modulation signal thus produced is used as the recording signal.
In the CD (Compact Disc) player systems, the demodulation system of the PCM digital signal is generally constructed such that a read out clock signal is fixed in frequency and the rotation of the recording disc is synchronized in phase with respect to the read out clock signal. Further, jitter is removed by reading out information which is temporarily stored in a memory in synchronism with the read out clock signal, which information is stored in the memory in synchronism with a write clock signal synchronized with a playback clock signal.
In the case of the above mentioned LDD, on the other hand, in the demodulation of the digital signal picked up from the playback signal, the playback signal is already synchronized with a reference (clock) signal for the video synchronization. Therefore, if in the PCM demodulation system, the data is read out from the memory using a different reference (clock) signal which is used in the general CD players, there will be inevitably a time difference between the playback video signal and the reproduced audio signal due to a slight phase difference between these two reference signals.
In addition, if the read out clock signal is advanced in phase with respect to the write clock signal, the memory may become empty. Conversely, if the write clock is advanced with respect to the read out clock, an overflow of the memory will occur.
Further, in the time of developing a compatible player system which is capable of playing both of the CD and LDD, various problems such as the aforementioned must be solved for using a demodulation system commonly for the processing of a CD output signal and the processing of an LDD output signal for the purpose of cost reduction.
Moreover, for the generation of the aforementioned write clock signal in accordance with the playback clock signal contained in the playback digital signal, it is general to use a PLL (Phase Locked Loop) circuit including a voltage controlled oscillator for generating the write clock signal and a phase comparator for generating a control voltage for the voltage controlled oscillator in accordance with a phase difference between the playback clock signal and the write clock signal. However, in such a time as a transitional time of a spindle servo system for driving a spindle motor from an unlocked state to a locked state, or as a time when a random access command such as the scan, search, or jump commaned is cancelled, the period for the lock-in of the PLL circuit tends to be prolonged because the playback clock signal is not stable before that time.
On the other hand, in some of the conventional CD players, an oscillator for controlling the rotation of the recording disc is provided separately. The frequency and the phase of the playback clock signal are also determined in accordance with the rotation of the recording disc. However, there can be errors of the frequency and the phase of an oscillation signal of the above mentioned oscillator for controlling the rotation of the recording disc, with respect to the frequency and the phase of the above mentioned read out clock signal generated by a read out clock signal oscillator due to a temperature change and so on. If the frequency of the read out clock signal becomes lower than the frequency of the writing signal which is determined by the frequency of the playback clock signal, the memorized data will remain in the memory due to the drop of the read out clock signal frequency. Therefore, it is necessary to provide a large enough memory capacity. Thus the size of the memory must be enlarged as the magnitude of an estimated error becomes large.